Conventional light level sensors normally incorporate photoelectric transducers which have an inherently slow response time, typically significantly greater than 20 milliseconds. The transducers produce a dc output signal, the level of which varies with the illumination level of incident radiation. Also, the response time of the complete sensors is normally damped to allow for transitory events, such as temporary cloud cover, and the total effective response time of the sensors may be fixed as high as 30 seconds.
Consequently, conventional types of light level sensors cannot distinguish between natural (ambient) light and artificial light that normally is modulated at a rate equal to the supply frequency or, in the case of lighting sources such as compact fluorescent lamps that have their own power supply, at a rate in the range 20 to 50 kHz. This means that the sensors must be shielded from the artificial light, a factor that sometimes escapes the attention of electricians and others who are responsible for siting the sensors, in order to prevent "confusion" arising from the existence of artificial light and to prevent the establishment of a slow ON-OFF switching cycle.
At least one attempt has been made to solve this problem by providing a sensor housing that incorporates a movable shield to hide the sensor from an adjacent artificial light source, but the present invention is directed to a more general solution of the problem by providing a sensor that detects for the presence of modulated light and uses this to distinguish between high light levels produced on the one hand by natural light and on the other hand by artificial light.